Use lock when popping requests on server shutdown
Use lock when popping requests on server shutdown

Doing this without a lock causes TSAN failures for quic.

There isn't much need to be clever here because this only impacts
shutdown performance, which doesn't really matter.

jboeuf/grpc,grpc/grpc,ncteisen/grpc,ncteisen/grpc,firebase/grpc,chrisdunelm/grpc,dgquintas/grpc,deepaklukose/grpc,mehrdada/grpc,ncteisen/grpc,grpc/grpc,vjpai/grpc,donnadionne/grpc,Vizerai/grpc,jtattermusch/grpc,pszemus/grpc,muxi/grpc,Vizerai/grpc,vjpai/grpc,stanley-cheung/grpc,thinkerou/grpc,ejona86/grpc,sreecha/grpc,firebase/grpc,stanley-cheung/grpc,adelez/grpc,ncteisen/grpc,thinkerou/grpc,simonkuang/grpc,kpayson64/grpc,donnadionne/grpc,pszemus/grpc,nicolasnoble/grpc,carl-mastrangelo/grpc,kpayson64/grpc,PeterFaiman/ruby-grpc-minimal,ejona86/grpc,dgquintas/grpc,jtattermusch/grpc,stanley-cheung/grpc,grpc/grpc,ctiller/grpc,thinkerou/grpc,kpayson64/grpc,mehrdada/grpc,donnadionne/grpc,fuchsia-mirror/third_party-grpc,deepaklukose/grpc,thinkerou/grpc,ncteisen/grpc,sreecha/grpc,fuchsia-mirror/third_party-grpc,vjpai/grpc,dgquintas/grpc,pszemus/grpc,muxi/grpc,nicolasnoble/grpc,Vizerai/grpc,muxi/grpc,muxi/grpc,jtattermusch/grpc,stanley-cheung/grpc,dgquintas/grpc,donnadionne/grpc,grpc/grpc,Vizerai/grpc,nicolasnoble/grpc,ctiller/grpc,kpayson64/grpc,nicolasnoble/grpc,jboeuf/grpc,grpc/grpc,ctiller/grpc,carl-mastrangelo/grpc,vjpai/grpc,sreecha/grpc,vjpai/grpc,firebase/grpc,muxi/grpc,thinkerou/grpc,sreecha/grpc,nicolasnoble/grpc,pszemus/grpc,muxi/grpc,donnadionne/grpc,simonkuang/grpc,jboeuf/grpc,donnadionne/grpc,ctiller/grpc,muxi/grpc,simonkuang/grpc,sreecha/grpc,firebase/grpc,firebase/grpc,ctiller/grpc,PeterFaiman/ruby-grpc-minimal,Vizerai/grpc,chrisdunelm/grpc,simonkuang/grpc,fuchsia-mirror/third_party-grpc,donnadionne/grpc,mehrdada/grpc,donnadionne/grpc,Vizerai/grpc,adelez/grpc,muxi/grpc,kpayson64/grpc,murgatroid99/grpc,PeterFaiman/ruby-grpc-minimal,sreecha/grpc,thinkerou/grpc,PeterFaiman/ruby-grpc-minimal,chrisdunelm/grpc,jboeuf/grpc,ejona86/grpc,fuchsia-mirror/third_party-grpc,dgquintas/grpc,jtattermusch/grpc,ejona86/grpc,pszemus/grpc,jtattermusch/grpc,mehrdada/grpc,jtattermusch/grpc,sreecha/grpc,grpc/grpc,grpc/grpc,carl-mastrangelo/grpc,nicolasnoble/grpc,nicolasnoble/grpc,sreecha/grpc,ejona86/grpc,geffzhang/grpc,Vizerai/grpc,ctiller/grpc,firebase/grpc,pszemus/grpc,grpc/grpc,fuchsia-mirror/third_party-grpc,dgquintas/grpc,thinkerou/grpc,carl-mastrangelo/grpc,geffzhang/grpc,simonkuang/grpc,stanley-cheung/grpc,thinkerou/grpc,deepaklukose/grpc,murgatroid99/grpc,adelez/grpc,simonkuang/grpc,fuchsia-mirror/third_party-grpc,stanley-cheung/grpc,ncteisen/grpc,mehrdada/grpc,murgatroid99/grpc,jtattermusch/grpc,pszemus/grpc,geffzhang/grpc,kpayson64/grpc,ncteisen/grpc,adelez/grpc,grpc/grpc,muxi/grpc,vjpai/grpc,vjpai/grpc,stanley-cheung/grpc,ejona86/grpc,PeterFaiman/ruby-grpc-minimal,muxi/grpc,adelez/grpc,ncteisen/grpc,firebase/grpc,chrisdunelm/grpc,vjpai/grpc,fuchsia-mirror/third_party-grpc,fuchsia-mirror/third_party-grpc,murgatroid99/grpc,kpayson64/grpc,donnadionne/grpc,murgatroid99/grpc,ejona86/grpc,ctiller/grpc,stanley-cheung/grpc,grpc/grpc,murgatroid99/grpc,jboeuf/grpc,simonkuang/grpc,deepaklukose/grpc,PeterFaiman/ruby-grpc-minimal,dgquintas/grpc,dgquintas/grpc,jtattermusch/grpc,nicolasnoble/grpc,jtattermusch/grpc,adelez/grpc,chrisdunelm/grpc,jtattermusch/grpc,ncteisen/grpc,nicolasnoble/grpc,Vizerai/grpc,geffzhang/grpc,chrisdunelm/grpc,stanley-cheung/grpc,jboeuf/grpc,deepaklukose/grpc,geffzhang/grpc,kpayson64/grpc,jtattermusch/grpc,mehrdada/grpc,pszemus/grpc,ejona86/grpc,vjpai/grpc,jtattermusch/grpc,thinkerou/grpc,fuchsia-mirror/third_party-grpc,stanley-cheung/grpc,chrisdunelm/grpc,mehrdada/grpc,thinkerou/grpc,nicolasnoble/grpc,murgatroid99/grpc,ctiller/grpc,murgatroid99/grpc,mehrdada/grpc,firebase/grpc,geffzhang/grpc,carl-mastrangelo/grpc,deepaklukose/grpc,kpayson64/grpc,simonkuang/grpc,murgatroid99/grpc,ejona86/grpc,adelez/grpc,ctiller/grpc,geffzhang/grpc,PeterFaiman/ruby-grpc-minimal,pszemus/grpc,donnadionne/grpc,ncteisen/grpc,pszemus/grpc,mehrdada/grpc,sreecha/grpc,ctiller/grpc,adelez/grpc,ncteisen/grpc,PeterFaiman/ruby-grpc-minimal,chrisdunelm/grpc,jboeuf/grpc,fuchsia-mirror/third_party-grpc,thinkerou/grpc,carl-mastrangelo/grpc,chrisdunelm/grpc,sreecha/grpc,dgquintas/grpc,jboeuf/grpc,mehrdada/grpc,firebase/grpc,chrisdunelm/grpc,carl-mastrangelo/grpc,stanley-cheung/grpc,dgquintas/grpc,sreecha/grpc,carl-mastrangelo/grpc,vjpai/grpc,jboeuf/grpc,firebase/grpc,sreecha/grpc,ejona86/grpc,jboeuf/grpc,carl-mastrangelo/grpc,vjpai/grpc,murgatroid99/grpc,mehrdada/grpc,ejona86/grpc,ncteisen/grpc,adelez/grpc,deepaklukose/grpc,carl-mastrangelo/grpc,kpayson64/grpc,simonkuang/grpc,Vizerai/grpc,stanley-cheung/grpc,nicolasnoble/grpc,mehrdada/grpc,geffzhang/grpc,thinkerou/grpc,carl-mastrangelo/grpc,PeterFaiman/ruby-grpc-minimal,muxi/grpc,jboeuf/grpc,ejona86/grpc,ctiller/grpc,deepaklukose/grpc,muxi/grpc,jboeuf/grpc,ctiller/grpc,chrisdunelm/grpc,Vizerai/grpc,nicolasnoble/grpc,vjpai/grpc,firebase/grpc,pszemus/grpc,pszemus/grpc,donnadionne/grpc,grpc/grpc,dgquintas/grpc,grpc/grpc,geffzhang/grpc,Vizerai/grpc,firebase/grpc,donnadionne/grpc,PeterFaiman/ruby-grpc-minimal,deepaklukose/grpc,kpayson64/grpc,carl-mastrangelo/grpc